Enhanced foreground mitigation in thermal SZ Compton-$y$ maps via polarization and deprojection

Abstract

Residual foreground contamination in thermal Sunyaev-Zeldovich (SZ) Compton-$y$ parameter maps ($y$-maps) arises mainly from Galactic emissions -- thermal dust and synchrotron radiation -- on large angular scales, and from cosmic infrared background (CIB) anisotropies on small scales. Unlike the thermal SZ effect, Galactic foregrounds are strongly polarized. Exploiting this distinction, we introduce a hybrid Needlet Internal Linear Combination (Hybrid NILC) method that combines Planck total-intensity and polarization frequency maps in the component-separation pipeline, thereby improving the suppression of residual Galactic emission while preserving the unpolarized SZ signal by leveraging the intrinsic $TE$ and $TB$ correlations of thermal dust and synchrotron. Using Planck PR4 data, we find that the Hybrid NILC $y$-map exhibits about $40\,\%$ lower cross-correlation with the IRAS dust tracer than the standard temperature-only Planck $y$-map, indicating reduced residual Galactic contamination. Simulations further indicate that, for future high-sensitivity surveys such as LiteBIRD, the Hybrid NILC will become increasingly effective at suppressing Galactic residuals. We further address small-scale extragalactic contamination by selectively deprojecting specific moments of the CIB using a Constrained Hybrid NILC variant, achieving an improved balance between CIB suppression and noise penalty compared to previous implementations in the literature. These novel approaches -- particularly the joint use of temperature and polarization in component separation -- offer a powerful framework for disentangling polarized and unpolarized signals.

Figures (14)

• Figure 1: Eigenvalues of the correlation matrix ${\bf \rm P}$ (Equation \ref{['eq:pearson']}), quantifying the degree of correlation between temperature and polarization channels primarily driven by Galactic foregrounds. Larger eigenvalues indicate stronger correlation, enabling greater Galactic foreground suppression in the $y$-map via the multi-Stokes Hybrid ILC. Left: Planck PR4 data (dash-dot green / circles) versus Planck-like simulations (dashed red / squares). The larger eigenvalues in PR4 data indicate stronger intrinsic temperature--polarization correlation of Galactic emission than captured in the simulations, suggesting improved Hybrid ILC performance on real data. Right: Planck-like simulations (dashed red / squares) versus low-noise simulations (blue dotted / triangles). Increased sensitivity enhances the effective temperature--polarization correlation of Galactic foregrounds, as the measured correlation becomes less diluted by instrumental noise.
• Figure 2: Top: Ratio of the total power spectra, $R_\ell^{{\rm tot}}$ (Equation \ref{['eq:cl_diff']}), between the $TQU$ and $T$$y$-maps for the Planck-like simulations (solid red) and the low-noise simulations (solid blue). Bottom: Ratio of the Galactic foreground residual power spectra, $R_\ell^{{\rm Gal}}$ (Equation \ref{['eq:cl_diff']}), between the $TQU$ and $T$$y$-maps for the same simulations. Dotted lines show null tests in which Galactic components are removed from the polarization $Q$ and $U$ channel maps prior to the $y$-map reconstruction. These results demonstrate that the suppression of Galactic foreground residuals in the $TQU$$y$-map occurs only when temperature and polarization foreground components are correlated, and that the level of suppression increases with improved sensitivity.
• Figure 3: $50^$^∘$\times 24^$^∘$$ strip around the Galactic plane, centred at Galactic coordinates ${(\ell,b)=(125^$^∘$,5^$^∘$)}$, showing Galactic residual maps (smoothed to $90'$) for the $T$ (top) and $TQU$ (bottom) $y$-map reconstructions in the low-noise simulation. The RMS values are $2.82\times10^{-7}$ and $2.02\times10^{-7}$, respectively, corresponding to about $30\%$ reduction in residual Galactic contamination for the $TQU$$y$-map in this region.
• Figure 4: Top: Input $y$-map showing three galaxy clusters in the Galactic plane region. Middle row: $T$$y$-map reconstruction (left) and $TQU$$y$-map reconstruction (right) for the low-noise simulation. Bottom row: Corresponding Galactic residual maps for the $T$ (left) and $TQU$ (right) reconstructions. Circled regions highlight fainter nearby clusters visible in the input map that are recovered in the $TQU$ reconstruction but not in the $T$ reconstruction.
• Figure 5: A $62.5^$^∘$\times 50^$^∘$$ region of the sky, centred at Galactic coordinates $(\ell,b)=(0^$^∘$,-45^$^∘$)$, showing the PR4 $T$$y$-map (left, PR4NILCymap) and the PR4 $TQU$$y$-map (right, this work). The two reconstructions appear visually very similar, demonstrating consistent recovery of the thermal SZ signal from galaxy clusters.